Blast furnaces for the production of pig iron are charged from the top with a stack of layers of burden material consisting of iron ore (sinter or pellet), coke, and fluxes such as limestone. Combustion is initiated and air is introduced or “blasted” upward as the oxidant for burning the coke, producing heat, and for smelting the iron from the ore. The fluxes combine with the silica in the ore so that it is removed as slag. The iron bearing ore provides the desired end product. The coke provides the fuel, acts as a reducing agent in the smelting process and also maintains porosity in the blast furnace stack for the combustion and exchange of heat and flow of iron downward. One of the primary limitations placed upon blast furnace throughput is the furnace working volume and, if the quantity of coke being charged can be reduced by introducing an acceptable alternative fuel; more of the working volume may be the iron ore so that a higher iron bearing material results in a higher output. In particular, skip charged furnaces which at present have a limit on the charge rate, can benefit from using less coke. However, the coke provides at least the important functions to provide the fuel for the process, to act as a reducing agent and to provide porosity in the stack so that the flow of the process can be maintained by preventing the stack from collapsing on itself and choking out the flow of air, heat and combustion, and iron material collection. The coke that that has the capability to act as a fuel, a reducing agent and also to provide the strength to maintain structural integrity during the process, and hence provide porosity in the blast furnace, has traditionally been produced by pyrolyzing metallurgical grade coal
Several substitutes for a portion of the coke have included injection of petroleum fuel products, heavy petroleum and natural gas as a fuel substitute. As a result of increases in petroleum cost, it is sometimes cost effective for a blast furnace to rely on all-coke operation that reduces that amount of injection of heavy oil.
In recent years several attempts have been made to inject crushed or pulverized coal in blast of air through the tuyeres of the blast furnace. By this technique, up to 20% of the coke charge has been replaced by coal injection, thus allowing higher production rates and reduced fuel costs. The cost of coke is generally about twice the cost of metallurgical coal used to make coke such that a significant cost savings can be realized.
Research and studies are currently under way on ways to enhance operation of blast furnaces by injection of an air carried pulverized fuel such as nodular coal or fine coal powder in a processes known generally known as Pulverized Coal Injection (PCI) to reduce the consumption of costly coke, petroleum fuel, or other fuel sources that have a higher cost than the coal. However, the pulverized fuel, such as particulate coal or fine coal, or the like, can have drawbacks in that it may have a low combustion rate as compared with other more expensive fuel such as natural gas or heavy oil, it may also contain other components such as moisture and ash that can interfere with the process and it may contain volatile components that can result in increased pollution.
In a heavy oil injection operation, the tip end of a burner is located in the vicinity of the boundary between a tuyere of a blast furnace and a blow pipe so as to burn the injected fuel completely in a raceway immediately downstream of the tuyere. However, if a slow burning pulverized coal fuel is injected at the same position, as the fast burning oil it is difficult to burn the pulverized coal fuel completely within the tuyere and raceway, resulting in lower combustion efficiency.
Moving the injection position further upstream has been considered as a way to improve the situation associated with slow burning injection coal. The longer distance gives the fuel a longer time to burn. Relocating the coal injection point is not a complete solution because pulverized coal fuels may also have a significant ash content (primarily silica), which ash will be fused by the combustion heat and will tend to deposit or accumulate on the inner surfaces of the longer blow pipe upon collision there against. This can narrow the blow passage and can make it difficult to continue a stable fuel injecting operation. There is also a possibility of the ash deposit destabilizing the hot air blowing through the tuyere. The amount of ash deposition or accumulation increases if the injecting position is located in a more upstream position.
It is known in the metallurgical arts to use the injection of coal dust or coal granules into a blast furnace to supply additional fuel for increasing the efficiency of smelting of iron ores for steel making in blast furnaces. It has been preferred by some to use granules instead of powder to burn in the blast furnace environment because the use of granules both reduces the costs associated with pulverizing coal to powder and also avoids the tendency for the powder to pack and clog passageways. One coal injection process has become known to some in the industry as Blast Furnace Coal Granule Injection (BFGCI).
It has been found that some of the problems associated with coal injection processes can be improved significantly by using a high grade coal sometimes known as metallurgical grade coal.
Metallurgical coal is the type of coal that can be used to produce metallurgical coke. Metallurgical grade coal is typically high grade bitumous coal. Metallurgical coal generally has a Free Swelling Index (FSI) of about 8 or 9. These types of coal have a high carbon content (more than about 60% and generally about 80%) corresponding to low volatile content of about 20% and low ash and impurities content, sometimes as low as 4 or 5%. Bitumous coal includes carbon compounds that are primarily composed of sp2 bonded carbon. Metallurgical coal also has a low or moderate moisture content and low ash content. The burning rate is generally lower than some of the other types of coal because the reaction rate for sp2 bonded carbon is slower than for other types of carbon compounds, for example carbonaceous material that has more than about 50% sp3 carbon bonds and thermal coal thermal coal having at leas more than 50% sp3 carbon bonds or that primarily has sp3 carbon bonds.
Thus, while metallurgical grade coal is used in making the coke, it has also been used in PCI for a number of reasons including efficiency and reduced pollution. The supply of such high grade metallurgical coal is relatively scarce compared to other types of coal or carbonaceous material and thus metallurgical coal is generally more expensive than other types of coal and many other types of carbonaceous.